Why do birds' eyes become redder with age? (14 March 2003)
We all know that eye colour is one of the few ageing characteristics available for the Dunnock Prunella modularis, and is also mentioned in the reference books (Svensson and Baker) for a few other species including Sparrowhawk Accipiter nisus and Collared Dove Streptopelia decaocto. This Collared Dove, caught this week, shows a classic 'ruby-red' eye, which they almost all have at this time of year.
Svensson mentions, on p.46 in the fourth (green) edition, that changes in iris colour might be useful for ageing a wide variety of species, and iris colour (along with leg and tongue/ mouth colours) is one of the characteristics to look for when trying to age 'adult' Reed Warblers as 5 or 6.
One of the most obvious species for ageing by iris colour is surely the Great Spotted Woodpecker Dendrocopos major. One caught this morning was an 'easy' age 5 (hatched 2002), still having a very dull brownish eye, shown on the left below. For comparison, in the centre is an adult, age 6, eye in February, and another 5 eye from January: this one was becoming quite reddish, but nothing like that of the adult. However, iris colour isn't mentioned by Baker for GRSWO, which seems odd, especially when he does cite it for two of the other Picidae - Wryneck Jynx torquilla (where it is actually much more variable than Baker suggests) and Green Woodpecker Picus viridis.
One thing that always intrigues me: with all of these disparate species (raptors, doves, woodpeckers, passerines), the iris colour always seems to become redder with age. Why is this?